8776: Upgrade Microsoft.CodeDom.Providers.DotNetCompilerPlatform to latest version

[BenedekFarkas]

Fixes #8776

Why do we need this package at all:

1. Dynamic Compilation, as in modify C# code and the module will be recompiled and loaded into memory when reloading the page - it's a useful developer feature.
2. VS Razor IntelliSense: The ASP.NET Compiler that ships with .NET Framework, even in the latest versions is an old one that doesn't support C# language features above version 5. The projects that have Razor files need this DLL to be loaded in memory (but AFAIR not necessarily strict about having it as a reference, just need to have some web.config entries).
3. Static Razor compilation: When building the solution with MvcBuildViews=true, Razor files are compiled in-memory as an extra validation step (but they remain as .cshtml files, so they aren't compiled into a DLL like in ASP.NET Core). This is the pickiest out of these 3 features and every project that has Razor files needs this DLL reference (and some configuration).

Further changes and details besides package update:

• Overrode /langversion:default that comes from the NuGet package upgrade in web.configs and set it to 7.3. The officially supported C# language version since .NET Framework 4.8 is 7.3 (and it probably won't go higher than that due to netstandard2.1). It is possible to build a project using a higher version (because it depends on which version your VS supports), but according to Microsoft it's not recommended to go higher than that due to potential runtime issues.
• Instead of having <LangVersion>latest</LangVersion> in csprojs for both the Release and Debug configuration, <LangVersion>7.3</LangVersion> is set just once regardless of configuration.
• "Downgraded" a small piece of src\Orchard.Web\Modules\Orchard.Email\Services\SmtpMessageChannel.cs to C# 7.3.
• We had <MvcBuildViews>false</MvcBuildViews> in the csprojs that use this package until now, we don't need it anymore.
• The CopyRoslynFilesToOutputFolder target from Orchard.Web.csproj is removed, because we're using the target with the same purpose (CopyRoslynCompilerFilesToOutputDirectory) supplied by the package.
• On top of the previous, Orchard.Web also defines RoslynToolPath explicitly (= where the tools are copied from). Without it the tools are only copied on the 2nd-3rd rebuild for some reason.
• Orchard.proj: Factored out a part of the Compile target into the DevCompile target and removed the BuildViews target.
  • For a simple local build (just to validate that the solution builds successfully, but don't want to run any tests), we don't actually need that second step that copies files to the output folder, so you can just use DevCompile instead of Compile.
  • CI builds (Test, Spec, etc.) that operate on the build output folder are unaffected, because Compile calls DevCompile.
  • The BuildViews target is not really necessary, just call any other target (Compile, DevCompile, Spec, etc.) with /p:MvcBuildViews=true instead.

[MatteoPiovanelli-Laser]

I have two questions: Why is this pinning the LangVersion? Do we need this package in so many projects?

[BenedekFarkas]

I have two questions: Why is this pinning the LangVersion? Do we need this package in so many projects?

Version pinning: The officially supported C# language version since .NET Framework 4.8 is 7.3 (and it probably won't go higher than that due to netstandard2.1). It is possible to build a project using a higher version (because it depends on which version your VS supports), but according to Microsoft it's not recommended to go higher than that due to potential runtime issues.

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Package reference: This whole saga of adding Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider to the solution/projects serves 3 purposes - the necessary configuration has some overlap between them.

1. Dynamic Compilation, as in modify C# code and the module will be recompiled and loaded into memory when reloading the page - it's a useful developer feature.
2. VS Razor IntelliSense: The ASP.NET Compiler that ships with .NET Framework, even in the latest versions is an old one that doesn't support C# language features above version 5. The projects that have Razor files need this DLL to be loaded in memory (but AFAIR not necessarily strict about having it as a reference, just need to have some web.config entries).
3. Static Razor compilation: When building the solution with MvcBuildViews=true, Razor files are compiled in-memory as an extra validation step (but they remain as .cshtml files, so they aren't compiled into a DLL like in ASP.NET Core). This is the pickiest out of these 3 features and every project that has Razor files needs this DLL reference (and some configuration).

[BenedekFarkas]

Made further changes and explained them in the PR description + added my previous answers there too, so they are all in one place.